Humans sometimes defecate at times of extreme fear due to the acute stress response (aka the "flight or fight" response).

When we are experiencing extreme fear, the sympathetic nervous system goes into overdrive and produces a state wherein we are better prepared to engage the source of the fear in a struggle or better prepared to flee from it.

A number of temporary physiological changes occur in the acute stress response. For example, more adrenaline is pumped through the system. This boosts heart and lung activity and thus aides physical functioning needed in fighting or running.

Other temporary physiological changes include dilation of blood vessels for muscles, constriction of blood vessels in parts of the body not needed for fighting or fleeing, liberation of nutrients needed for muscular action, inhibition of tear glands and salivation, dilation of the pupil, inhibition of stomach and intestinal action, inhibition of erection in males, and relaxation of elimination control. Thus, under circumstances of acute stress such as in extreme fear, the bladder and bowels can "let go".

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to [email protected]